A major cause of failure of concrete structures is corrosion of their reinforcement metal. The corrosion rate of embedded reinforcement bars (rebars) is an important parameter in assessing the remaining life of a concrete structure and the risk of its failure.
Efforts have been made to use electrochemical methods to measure the corrosion rate of rebars in concrete structures. However, these methods are challenging because the measurement area of the metal is difficult to define. When an electrochemical measurement is attempted from the rebar through a counter electrode, the measured current may not only be from corrosion reaction on metal near the counter electrode, but also from corrosion reactions occurring far away from the counter electrode. It is difficult to determine the corroding area that is contributing to the measured current, and thus difficult to measure a corrosion rate.
The concept of a guarded ring electrode for concrete-embedded rebar has been used to confine the current from the counter electrode to a certain area of the rebar. In a guarded ring electrode system, the counter electrode is surrounded by a large ring electrode, and both are flush mounted in an insulator. During measurement, the sensing surfaces of the flush-mounted counter electrode and guarded ring electrode are placed onto the concrete surface near the rebar and the rebar is used as the working electrode. The guarded ring electrode and the counter electrode are controlled at the same potential against the rebar, and the currents from the two electrodes to the rebar are measured separately. Because the guarded ring electrode is relatively large and creates a near-parallel electrical field around the counter electrode, the current from the counter electrode is confined to a window corresponding to the surface area of the counter electrode. Therefore, the total surface area of the rebar that is responsible for the current measured from the counter electrode is the area within the window of the counter electrode. This concept works well for simple structures such as a single rebar with a known diameter underneath the counter electrode, but is not accurate for complicated structures with many rebars connected together.
Because of the difficulties in measuring the corrosion rate of actual rebars, probes made of surrogate materials are often used to estimate the corrosion rate of the actual rebars. However, unless the probes are embedded in the concrete at the same time as the rebar, and have similar corrosion susceptibility, and have a similar corrosive environment, the corrosion rate of the probe may not be the same as the corrosion rate of the concrete rebar. In addition, corrosion probes are usually operated independently with dedicated electronic instruments and can only be installed in a limited number of locations in a concrete structure.